Computers study material

Basics of Computers

What is a computer?

A computer is an electronic device that manipulates information, or data. It has the ability to store, retrieve, and process data. You probably already know that you can use a computer to type documents, send email, play games, and browse the Web. You can also use it to edit or create spreadsheets, presentations, and even videos.

Hardware :

Hardware is any part of your computer that has a physical structure, such as the keyboard or mouse. It also includes all of the computer's internal parts.

Software :

Software is any set of instructions that tells the hardware what to do. It is what guides the hardware and tells it how to accomplish each task. Some examples of software include web browsers, games, and word processors.

Computers are usually categories into three general categories:

1) Supercomputer :The fastest, largest, most powerful and most expensive computer..

2) Mainframe Computer :This is a little smaller and less powerful than the supercomputer, but, like the supercomputer it is also expensive.

3) Personal Computer (PC): This is the computer that most people use in their daily lives. This computer is much smaller, less powerful and less expensive than the supercomputer and the mainframe computer.

Generations of Computers:

First Generation of Computers (1942-1955) :

The first computers used vacuum tubes for circuitry and magnetic drums for memory, and were often enormous, taking up entire rooms. They were very expensive to operate and in addition to using a great deal of electricity, the first computers generated a lot of heat, which was often the cause of malfunctions. First generation computers relied on machine language, the lowest-level programming language understood by computers, to perform operations, and they could only solve one problem at a time, and it could take days or weeks to set-up a new problem. Input was based on punched cards and paper tape, and output was displayed on printouts. The UNIVAC and ENIAC computers are examples of first-generation computing devices. The UNIVAC was the first commercial computer delivered to a business client, the U.S. Census Bureau in 1951.

Second Generation Computers (1955-1964) :

Transistors replace vacuum tubes and ushered in the second generation of computers. The transistor was invented in 1947 but did not see widespread use in computers until the late 1950s. The transistor was far superior to the vacuum tube, allowing computers to become smaller, faster, cheaper, more energy-efficient and more reliable than their first-generation predecessors. Though the transistor still generated a great deal of heat that subjected the computer to damage, it was a vast improvement over the vacuum tube. Second-generation computers still relied on punched cards for input and printouts for output. Second-generation computers moved from cryptic binary machine language to symbolic, or assembly, languages, which allowed programmers to specify instructions in words. High-level programming languages were also being developed at this time, such as early versions of COBOL and FORTRAN. These were also the first computers that stored their instructions in their memory, which moved from a magnetic drum to magnetic core technology. The first computers of this generation were developed for the atomic energy industry.

Third Generation Computers (1964-1975) :

The development of the integrated circuit was the hallmark of the third generation of computers. Transistors were miniaturized and placed on silicon chips, called semiconductors, which drastically increased the speed and efficiency of computers. Instead of punched cards and printouts, users interacted with third generation computers through keyboards and monitors and interfaced with an operating system, which allowed the device to run many different applications at one time with a central program that monitored the memory. Computers for the first time became accessible to a mass audience because they were smaller and cheaper than their predecessors.

Fourth Generation Computers (1975-Present) :

The microprocessor brought the fourth generation of computers, as thousands of integrated circuits were built onto a single silicon chip. What in the first generation filled an entire room could now fit in the palm of the hand. The Intel 4004 chip, developed in 1971, located all the components of the computer—from the central processing unit and memory to input/output controls—on a single chip. In 1981 IBM introduced its first computer for the home user, and in 1984 Apple introduced the Macintosh. Microprocessors also moved out of the realm of desktop computers and into many areas of life as more and more everyday products began to use microprocessors. As these small computers became more powerful, they could be linked together to form networks, which eventually led to the development of the Internet. Fourth generation computers also saw the development of GUIs, the mouse and handheld devices.

Fifth Generation Computers (Present & Beyond) :

Fifth generation computing devices, based on artificial intelligence, are still in development, though there are some applications, such as voice recognition, that are being used today. The use of parallel processing and superconductors is helping to make artificial intelligence a reality. Quantum computation and molecular and nanotechnology will radically change the face of computers in years to come. The goal of fifth-generation computing is to develop devices that respond to natural language input and are capable of learning and self-organization.

Shortcut keys
Shortcut Keys Description
Alt + F File menu options in current program.
Alt + E Edit options in current program
Alt + Tab Switch between open programs
F1 Universal Help in almost every Windows program.
F2 Rename a selected file
F5 Refresh the current program window
Ctrl + N Create a new, blank document in some software programs
Ctrl + O Open a file in current software program
Ctrl + A Select all text.
Ctrl + B Change selected text to be Bold
Ctrl + I Change selected text to be in Italics
Ctrl + U Change selected text to be Underlined
Ctrl + F Open find window for current document or window.
Ctrl + S Save current document file.
Ctrl + X Cut selected item.
Shift + Del Cut selected item.
Ctrl + C Copy selected item.
Ctrl + Ins Copy selected item
Ctrl + V Paste
Shift + Ins Paste
Ctrl + K Insert hyperlink for selected text
Ctrl + P Print the current page or document.
Home Goes to beginning of current line.
Ctrl + Home Goes to beginning of document.
End Goes to end of current line.
Ctrl + End Goes to end of document.
Shift + Home Highlights from current position to beginning of line.
Shift + End Highlights from current position to end of line.
Ctrl + Left arrow Moves one word to the left at a time.
Ctrl + Right arrow Moves one word to the right at a time.
Ctrl + Esc Opens the START menu
Ctrl + Shift + Esc Opens Windows Task Manager
Alt + F4 Close the currently active program
Alt + Enter Open the Properties for the selected item (file, folder, shortcut, etc.)
Computer Network

What is a Computer Network

A computer network consists of two or more autonomous computers that are connected together in order to share resources (files, printers, modem etc), share application, allow electronic communication etc. A computer network includes the network operating system in the client and server machines, the cables, which connect different computers and all supporting hardware in between such as bridges, routers and switches. In wireless system, antennas and towers are also part of the network.

Types of Computer Network:

Computer network are generally classified according to their structure and the area

Local Area Network (LAN):-

In LAN, computers are connected locally within a room, building or in a small area. LAN can range from simple (two computers) to complex (hundred of computers and peripherals).LANs connect workstations and personal computers. User can share data as well as devices like printers, fax etc and can use LAN to communicate with each other, by sending e-mail or chatting. Examples of LAN Technology: Ethernet, Token Ring, and Fiber Distributed Data Interconnect (FDDI).

Characteristics of LAN are:

   Configured to small area i.e. it connects several devices over a distance of 5 to 10 KM.

    High Speed

   Low error rates

    Data and hardware sharing between users.

   Operates at speed ranging from 10Mbps to 100 Mbps. Now a day’s 1000Mbps are available

Network Topology:

Topology is physical arrangement of computer system in a network. Or refers to the shape of networks. Some of the most common network topologies are

    Bus topology

In bus topology, all devices are connected to a central cable, called the bus or backbone. The bus topology connects workstations using a single cable. Each workstation is connected to the next workstation in a point to point fashion. All workstation connect to the same cable. An extension to the bus topology is tree topology.

Advantage of Bus topology

Installation is easy and cheap. Connections are simple and easy to use. Data can be transmitted from the both sides.

Disadvantage of Bus topology

A single fault in the cable stops all transmission. Fault identification is difficult.


Ring topology:

In ring topology, all devices are connected to one another in the shape of a closed loop; so that each device is connected directly to each other i.e. the ring topology connects workstations in a closed loop. Data is transmitted around the ring in one direction only. Each station passing on the data to the next station till it reaches its destination. The common implementation of this topology is token ring.

Advantage of Ring topology

Easy to install and modify the network and fault isolation is simplified.

Disadvantage of Ring Topology

Adding or removing computer disrupts the entire network. A break in the ring can stop the transmission in the entire network. Finding fault is difficult.


Star Topology:

Star topology uses a central hub through which, all components are connected. A central hub is the host computer and at the end of each connection is a terminal. Communication on the connecting links between the stations and the central station of star topology can be bi-directional and are point to point.

Advantage of Star Topology:

Expansion or modification is easy. Single computer failure does not affect the network.

Disadvantage of Star Network:

Failure in the central hub brings the entire network to halt.


Tree topology:

Tree topology is LAN topology in which only one route exists between any two nodes on the network. It is similar to the star topology but the nodes are connected to the secondary hub, which in turn is connected to the central hub.

Advantage of Tree Topology:

Installation and configuration of tree network is easy. Less expensive when compared to mesh topology.

Disadvantage of tree Topology:

Failure in central hub brings the entire network to a halt. More cabling is required when compared to bus topology.


Mesh topology:

Devices are connected with many redundant interconnections between network nodes. In a well connected topology, every node has a connection to every other node in the network. Mesh topology are used in critical connection of host computers [typically telephone exchange]. Alternate path allow each computer to balance the load to other computer systems in the network by using more than one connection path available.

Advantage of Mesh topology:

Failure in one of the computers does not affect the entire network. Privacy between computers is maintained as messages travel along dedicated path.

Disadvantage of Mesh Topology:

Amount of caballing required is high. A large number of I/O ports are required.


What is Internet

The Internet is a worldwide network of networks. It can be defined as a global network of over millions of smaller heterogeneous computer networks consists of a set of national, regional and private. The internet is the common language whereby dissimilar computers with various operating systems, are able to communicate with each other, using a standard set of protocols. The Internet is the world's largest computer network, a distinction it has earned by virtue of being a "network of networks." The Internet is an outgrowth of a network (ARPANET) established roughly a quarter-century ago to meet the needs of researchers working in the defense industry in the United States and a few of their colleagues in other countries. The ARPANET grew slowly, from a handful of computers in 1971 to more than 1000 in 1984. Working with the ARPANET researchers came to regard high-speed computer networks as an indispensable tool for academic research in all fields, and in 1986 the US National Science Foundation established NSFNET to provide network connections to more research institutions and improve international network cooperation. in 1987, the Internet served more than 10,000 computers. By 1989 the network had grown to more than 100,000.

Evolution of Internet

The origin of Internet can be traced to a U.S. Department of Defense (DoD) organization called Advanced Research Projects Agency (ARPA).

ARPA developed a four node packet switching. network called ARPANET in 1969. The network was intended to support the military research on fault tolerant computer networks. DoD wanted to ensure a reliable data transfer in the event of a nuclear war, even if parts of the network has been destroyed.

As the years pass by, the network grew in size and by 1st January 1983, the ARPANET no longer remained an experimental network but its control was passed over to Defense Communications Agency (DCA).

The network then became available for the academic research, government employee and contractors.

The world's fastest and most powerful computers were made available to an academic and scientific community.

In 1990 the ARPANET was officially decommissioned.

Some of the major networks contributing to the growth of the internet are as follows:


     USENET (User's network)

     CSNET (Computer science network)

     BITNET (Because it's time network)

     NSFNET (National science foundation network)

     WWW (World Wide Web)

      NREN (National research and education network)


  It is said that the Internet is growing at a rate of 20% per month. The data speeds have gone up considerably, which makes the access even faster.

  The event which started as a military assistance program is now largely a private enterprise.

Services on the Internet

The services that are available on the Internet can be classified into two categories:

     Communication services (electronic mail (e-mail))

     Information retrieval services (Web (pages with links and multimedia content of its Web sites))

Applications of Internet

Traditionally the internet has the following applications:



     Remote login

     File transfer

     World wide web (www).

TCP/IP model


Like OSI network model, TCP/IP also has a network model. TCP/IP was on the path of development when the OSI standard was published and there was interaction between the designers of OSI and TCP/IP standards. The TCP/IP model is not same as OSI model. OSI is a seven-layered standard, but TCP/IP is a four layered standard. The OSI model has been very influential in the growth and development of TCP/IP standard, and that is why much OSI terminology is applied to TCP/IP. The following figure compares the TCP/IP and OSI network models.


As we can see from the above figure, presentation and session layers are not there in TCP/IP model. Also note that the Network Access Layer in TCP/IP model combines the functions of Data link Layer and Physical Layer.


Application layer is the top most layer of four layer TCP/IP model. Application layer is present on the top of the Transport layer. Application layer defines TCP/IP application protocols and how host programs interface with Transport layer services to use the network.

Application layer includes all the higher-level protocols like DNS (Domain Naming System), HTTP (Hypertext Transfer Protocol), Telnet, SSH, FTP (File Transfer Protocol), TFTP (Trivial File Transfer Protocol), SNMP (Simple Network Management Protocol), SMTP (Simple Mail Transfer Protocol) , DHCP (Dynamic Host Configuration Protocol), X Windows, RDP (Remote Desktop Protocol) etc.


Transport Layer is the third layer of the four layer TCP/IP model. The position of the Transport layer is between Application layer and Internet layer. The purpose of Transport layer is to permit devices on the source and destination hosts to carry on a conversation. Transport layer defines the level of service and status of the connection used when transporting data.

The main protocols included at Transport layer are TCP (Transmission Control Protocol) and UDP (User Datagram Protocol).


Internet Layer is the second layer of the four layer TCP/IP model. The position of Internet layer is between Network Access Layer and Transport layer. Internet layer pack data into data packets known as IP datagram’s, which contain source and destination address (logical address or IP address) information that is used to forward the datagram’s between hosts and across networks. The Internet layer is also responsible for routing of IP data grams.

Packet switching network depends upon a connectionless internetwork layer. This layer is known as Internet layer. Its job is to allow hosts to insert packets into any network and have them to deliver independently to the destination. At the destination side data packets may appear in a different order than they were sent. It is the job of the higher layers to rearrange them in order to deliver them to proper network applications operating at the Application layer.

The main protocols included at Internet layer are IP (Internet Protocol), ICMP (Internet Control Message Protocol), ARP (Address Resolution Protocol), RARP (Reverse Address Resolution Protocol) and IGMP (Internet Group Management Protocol).


Network Access Layer is the first layer of the four layer TCP/IP model. Network Access Layer defines details of how data is physically sent through the network, including how bits are electrically or optically signaled by hardware devices that interface directly with a network medium, such as coaxial cable, optical fiber, or twisted pair copper wire.

The protocols included in Network Access Layer are Ethernet, Token Ring, FDDI, X.25, Frame Relay etc.

The most popular LAN architecture among those listed above is Ethernet. Ethernet uses an Access Method called CSMA/CD (Carrier Sense Multiple Access/Collision Detection) to access the media, when Ethernet operates in a shared media. An Access Method determines how a host will place data on the medium.

IN CSMA/CD Access Method, every host has equal access to the medium and can place data on the wire when the wire is free from network traffic. When a host wants to place data on the wire, it will check the wire to find whether another host is already using the medium. If there is traffic already in the medium, the host will wait and if there is no traffic, it will place the data in the medium. But, if two systems place data on the medium at the same instance, they will collide with each other, destroying the data. If the data is destroyed during transmission, the data will need to be retransmitted. After collision, each host will wait for a small interval of time and again the data will be retransmitted.

Memory management

Memory management:

Memory is part of the computer where programs and data are stored electronically. The data and programs are stored at two different levels:

     Main (internal) Memory

     Auxiliary (External) memory


The different kinds of main memory are:

     1. RAM(Random Access Memory)

     2. ROM(Read Only Memory)



The RAM of the computer is divided into many locations, which are uniquely numbered or addressed. Information and data are stored in these memory locations and are retrieved randomly from these locations. RAM contains data that needs to be processed and the instructions, which are used for processing. It is a read and write memory. It is a volatile and temporary memory.

Two types of RAM.:

     1. Static RAM is called static because it will continue to hold information without refreshment i.e it does not required refreshment

     2. Dynamic RAM is called dynamic because it require refreshment It loose information with time.


RAM is also called as volatile memory, as the contents of RAM are erased when the computer is switched off. The size of a computer is measured in terms of the number of storage locations in memory . It contains the information necessary to startup you computer and the basic instructions that operate it. The storage elements in the ROM contain certain pre-coded instructions, which are not available to the users. These storage locations are only read and cannot be erased or changed. So, it is also known as non-volatile memory. Certain ROM chips are available that can be erased.


It stands for Programmable read Only Memory. These ROMs become permanent if they are programmed. Once the chip is programmed, the information recorded cannot be changed.


It stands for Erasable Programmable Read Only Memory. By using a special process of exposing to ultraviolet light, the original information can be erased. Using a special programmer facility, the chip can be programmed to record new different information.


It stands for Electrically Erasable Programmable Read Only Memory. This memory can be programmed and erased by electrical signals.


As RAM has the limited storage capacity and is also not permanent, external storage devices supplement computer systems. They accept data or program instructions from the processor: retain them and can be accessed as needed to compute a processing task. They are called disk drive. Data stored in a disk can be directly accessed. Every disk has a directory of what is stored in the disk. The directory contains the name given to the data file and its storage location (address). Whenever the processor needs data, the read/write head of the disk drive identifies the disk address of data file. It moves to the specified track and then to the proper sector where the data is file. It moves to the specified track and then to the proper sector where the data is stored. The different types of storage devices are


It is a medium for magnetic recording, made of a thin magnetizable coating on a long, narrow strip of plastic film. It was developed in Germany, based on magnetic wire recording. Devices that record and play back audio and video using magnetic tape are tape recorders and video tape recorders. A device that stores computer data on magnetic tape is a tape drive (tape unit, streamer).


Hard Disk:

Hard Disk is the main computer storage device, which is fast, reliable and stores a lot of information. A non-removable device is used to store information in the computer so that it can be accessed later. It consists of several platters mounted on a spindle that are made to rotate in a very high speed. When multiple disks are packed together, a number of read and write heads are used to write or retrieve data. Hard disk is the fixed disk, which cannot be removed from the disk drive.

Floppy Disk:

Floppy disk is a thin sheet of magnetized plastic inside a protective covering where the data is stored. Floppy disks come in two sizes: 5 inch and 3 inch squares. The floppy disk is made to rotate and read /write head is in contact with the spinning disk. Data is written as dry magnetic spots on the disk surface. While writing new data, one disk erases data previously stored at the novation, but otherwise magnetic spots remains indefinitely reading by the magnified spots passing index read/write head. Floppy drive is an electronic device that can read and write information onto these disks. Floppy drives are part of the computer and they also come in two sizes depending on which size of disk they can read. Floppy disks are used mainly for software distributing, transporting files and creating backups.


On an optical disk data is seconded by burning a permanent pattern into the surface of the disk by means of a high precision laser rays. The optical disk is the type of CD-ROM on which you can both read for, and write Once/Read Many. You can write only once onto a WORM optical disk. It cannot be erased or over-written. After writing the information on it, you can only read it for as many times as you like.


Flash memory is non-volatile memory that is an integrated circuit that does not need continuous power to retain the data. It is much more expensive than magnetic storage. However, flash memory is widely used with car radios, cell phones, digital cameras, PDAs, MP3 players, and printers.



E-mail is the backbone of today's communication and it is a convenient medium for communications for all segments of users. First E-mail was sent in 1971 by an engineer Ray Tomlinson. Before this, email was limited to users within a single machine. Tomlinson's was able send messages to other machines on the Internet, using the @ sign to designate the receiving machine. Email messages are normally plain text messages, with or without attachments.

E-Mail server:

An E-Mail server is an application that receives incoming e-mail from local users and remote senders and forwards outgoing e-mail for delivery.


MS Exchange Server, SendMail, QMail etc. All recent E-mail servers work on Simple Mail Transfer Protocol (SMTP).

E-Mail clients:

E-Mail clients are programs which are used to create, send, receive, and view e-mails.


Mozilla Thunderbird, MS Outlook etc. .

All recent versions of E-mail clients allow messages to be formatted in plain text or.HTML. .


Some of the e-mail threats are explained below.


Normally all E-mail messages (including user-ids and passwords) are transmitted between computer and E-mail servers as plain text. This is not secure and anyone who can seize this can read your email and obtain your user-ids, passwords and sensitive E-mail content.

Spam E-mails

Spam is flooding the internet with many copies of unsolicited bulk E-mail messages. Spam E-mails are usually commercial advertisements like replica watches, cheap drugs, get rich quick and other dubious products. Spam E-mails decreases productivity and increases the cost of email use. Many legitimate E-mails are also filtered by spam filters.

E-mail Viruses and Worms:

Many viruses and worms are spread as attachment of e-mails. Once your computer is infected, the Viruses and Worms may spread your network computers also. These malwares can send your private information to attackers, destroy your precious data or disrupt productivity. Your E-mail server should be equipped with strong anti-virus application and spam filters to stop spam, viruses, and other unwanted content before they reach your infrastructure and users. Anti-virus application and spam filters should actively protect you against email bombs and other attacks, and greatly reduce the impact of spam and viruses.

E-Mail Bombs:

Email Bombs are a type of Denial of Service (DoS) attack which target mail servers. “Email bombs” occur when you receive a large number of E-mail messages in a very short time to overflow the mailbox or overload the server. .


Your E-mail server should be equipped with a trusted and updated anti-virus application and spam filters to stop spam, viruses, and other unwanted content before they reach your infrastructure and users. Anti-virus application and spam filters should actively protect you against email bombs and other attacks, and greatly reduce the impact of spam and viruses. Most popular encryption methods available for E-mails are Secure/Multipurpose Internet Mail Extensions (S/MIME) and PGP.

Secure/Multipurpose Internet Mail Extensions (S/MIME):

MIME (Multi-Purpose Internet Mail Extensions) is an extension of Simple Mail Transfer Protocol (SMTP). MIME (Multi-Purpose Internet Mail Extensions) provides the ability to transfer different types of data formats like audio, video, images, compressed files, and applications etc as attachments. The MIME (Multi-Purpose Internet Mail Extensions) header is inserted at the beginning of the E-mail, and the E-mail clients use the MIME (Multi-Purpose Internet Mail Extensions) information to determine which program associated with the attached file. MIME does not provide any security to the E-mails.

S/MIME (Secure/Multipurpose Internet Mail Extensions) is a protocol that adds digital signatures and encryption to Internet MIME (Multipurpose Internet Mail Extensions) messages. S/MIME (Secure/Multipurpose Internet Mail Extensions) provides authentication, message integrity and non-repudiation of origin (using digital signatures) and privacy and data security (using encryption) to E-Mails. E-mail Messages are encrypted using a symmetric encryption and a public-key algorithm is used for key exchange and digital signatures. DES, 3DES, or RC2 encryption can be used for encryption.

PGP (Pretty Good Privacy):

PGP (Pretty Good Privacy) is another encryption method for E-mails. PGP (Pretty Good Privacy) uses both symmetric encryption and asymmetric encryption. PGP uses symmetric encryption algorithm to encrypt the E-mail message to be sent. After encrypting the E-mail message PGP (Pretty Good Privacy) encrypts the symmetric key used to encrypt the E-mail message with the public key of the intended recipient. The encrypted key and E-mail message are then sent to the intended recipient. At The receiving side, PGP (Pretty Good Privacy) will first decrypt the symmetric key with the private key supplied by the recipient and will then use the decrypted symmetric key to decrypt the E-mail message.